1. Field of the Invention
The present invention relates to a digital recording and/or reproduction apparatus of a video signal, and more particularly, to a recording apparatus for carrying out bit rate reduction coding of a video signal and digitally recording the coded video signal on a recording medium, a reproduction apparatus for digitally reproducing a video signal at a changeable speed from a recording medium having such video signal recorded thereon, and a recording/reproduction apparatus having such digital recording and reproduction function.
2. Description of the Background Art
In recent years, an apparatus has been developed for recording digitally a video signal on a recording medium and reproducing digitally the same from the recording medium. One such typical apparatus is the so-called digital video tape recorder (referred to as "digital VTR" hereinafter) which is available for practical use. In such a conventional digital VTR, a digital video signal which is in a bit rate reduction coded (bandwidth compressed) state for the purpose of reducing the enormous amount of information to a level suitable for recording and reproduction is recorded on a magnetic tape and reproduced therefrom using a rotary head.
There are two methods of such bit rate reduction coding, that is, one method of merely controlling the code amount of video information constituting the entire screen, and another method of dividing the entire screen into a plurality of blocks and controlling the code amount so that the video information constituting each block has a fixed length. As will be described afterwards, the latter method is suitable as a bit rate reduction coding method for a digital VTR having reproduction function at a changeable speed.
FIG. 1 is a block diagram schematically showing a structure of a conventional digital VTR using the above mentioned latter bit rate reduction coding method. FIG. 2 schematically shows the manner of dividing screen into blocks according to such a bit rate reduction coding method. FIG. 3 schematically shows the data arrangement in tracks on a tape. Such a conventional digital VTR is disclosed in Japanese Patent Laying-Open No. 2-220270, for example.
Referring to FIG. 1, at the time of recording, a digital video signal provided from a video signal source not shown is supplied to a block shuffling circuit 101 included in a digital VTR. Block shuffling circuit 101 divides the digital video signal into blocks each having a certain size to rearrange the video signal data on the basis of such block.
More specifically, referring to FIG. 2, the entire rectangular represents one whole screen. The entire screen is divided into six regions of A, B, C, D, E and F. Each region is further divided into small rectangular units (small blocks), where small blocks in each region are indicated by numbers such as 1, 2, 3, . . . . For example, region A is formed of a plurality of small blocks of A1, A2, A3, . . . , and region B is formed of the same number of small blocks of B1, B2, B3, . . . . The same can be said for the remaining regions C, D, E and F.
Block shuffling circuit 101 divides the supplied digital video signal constituting an entire screen into the above-described rectangular units (small blocks). Then, the digital video signal is rearranged on a small block basis in such arrangement that small blocks are sequentially derived one by one from respective regions of A-F in ascending order, such as A1, B1, C1, D1, E1, F1, A2, B2, C2, D2, E2, F2, A3, . . . . Each group of small blocks of a corresponding number read out from the respective six regions A-F, for example a group of (A1, B1, C1, D1, E1, F1), a group of (A2, B2, C2, D2, E2, F2) . . . , is defined as one "large block".
The digital video signal rearranged as described above is applied to a bit rate reduction coding circuit 102 where bit rate reduction coding of data on the basis of the above-described large block is carried out to realize an amount of information suitable for recording onto a tape. More specifically, a video signal of each small block entered into bit rate reduction coding circuit 102 is subjected to the well-known orthogonal transform coding, and then subjected to a variable length coding process according to the information amount of each small block so that the coded amount of a large block becomes constant. As a result, reduction of the data amount, i.e. bit rate reduction coding is realized. The data of the plurality of small blocks constituting each large block are gathered together to be provided as a block data of a fixed length to be supplied to an error correction coding circuit 103.
Error correction coding circuit 103 adds an error correction code (parity) to the block data of a fixed length subjected to bit rate reduction coding, which is supplied to a synchronizing signal-ID signal applying circuit 104. Synchronizing signal-ID signal applying circuit 104 adds a synchronizing signal and an ID signal to the supplied video signal to provide the same to a modulation circuit 105.
Modulation circuit 105 modulates the supplied signal while suppressing the direct current (DC) component thereof to record the same on the above-described fixed length block basis on a magnetic tape 107 as a recording medium via a magnetic head 106. FIG. 3 shows the recording manner on such a tape. Each track is constituted by a plurality of block data of a fixed length M1, M2, M3, M4, . . . .
At the time of reproduction, recorded data M1, M2, M3, . . . on tape 107 such as that shown in FIG. 3 are reproduced via a magnetic head 108 to be demodulated by a demodulation circuit 109. The demodulated video signal is supplied to a synchronizing signal-ID signal detection circuit 110 where detection of a synchronizing signal and an ID signal is carried out. Then, the video signal is applied to an error correction decoding circuit 111.
Error correction decoding circuit 111 carries out error correction to the applied video signal, and variable length decoding and inverse orthogonal transform process for data of each of small blocks forming the fixed length block to restore the original data of each small block. The restored data is supplied to a decoding-concealment circuit 112 where decoding and concealment of a video signal are carried out for the portion of data where error correction could not be carried out in error correction decoding circuit 111.
The data subjected to an error correction and decoding process is supplied to a block de-shuffling circuit 113 where a inverse rearrangement of the rearrangement by block shuffling circuit 101 of the recording system is carried out. As a result, the video signal of the original data arrangement at the time of recording is reproduced. The data signal of block de-shuffling circuit 113 is output appropriately to be provided to a monitor device and the like not shown.
In a conventional digital VTR shown in FIGS. 1-3, a plurality of small blocks that are located distance from each other on a screen (for example, A1, B1, C1, D1, El, F1) are gathered together to form a large block, and control of the code amount is carried out on the basis of this large block to form a data block of a fixed length (for example M1) to be recorded on a tape. Therefore, corresponding small blocks (for example A1 and A2, B1 and B2) of two data of fixed length recorded adjacent to each other on a tape (for example M1 and M2) are the small blocks adjacent to each other on the screen. The deviation of the amount of information of video data is considered to be necessarily small between two adjacent small blocks on the screen. Therefore, deviation in each information amount of fixed length data (M1, M2, . . . ) recorded adjacent to each other on a tape is less likely to occur, so that the video signal can be transmitted with the desired quality of picture, that is, efficiency of bit rate reduction coding can be improved.
More specifically, it is necessary to allocate the code amount according to the information amount of a picture in order to code the video data while maintaining the desired picture quality. On the other hand, it is also required to make the code amount after coding constant in each large block. As a result, if the deviation in the information amount of each large block is large, the data of a large block having such large information amount is discarded and it becomes difficult to maintain sufficient picture quality. If the code length of the fixed length block is made large to correspond to that of the block having the largest amount of information in order to maintain sufficient picture quality, on the other hand, the code length of the large block having a smaller amount of information is also increased unnecessarily and it becomes difficult to carry out bit rate reduction coding with the improved efficiency. Accordingly, in the conventional digital VTR, the deviation in the information amount of each fixed length block was made small in the above described manner to carry out bit rate reduction coding with the improved efficiency.
In addition, when the video data is recorded on the basis of small block constituted by a plurality of pixels and such recorded data is reproduced at a high speed, small blocks belonging to different fields (frames) are usually reproduced adjacent to each other on the same screen and a boundary between such small blocks can be visually recognized in such a case. If a large number of small blocks belonging to different fields (frames) are reproduced in the minutely mixed manner, such boundaries are also generated minutely, resulting in the reproduced picture which appears as if a mosaic pattern processing has been applied.
Therefore, by maintaining continuity of data on a screen between fixed length blocks (for example M1 and M2) recorded adjacent to each other on a tape, data continuity on a screen can be ensured even at the time of high speed reproduction such as in high speed search mode to prevent a mosaic pattern appearing on a screen, whereby good visual quality of the reproduced picture can be ensured.
The manner of block division of a screen in the above-described conventional VTR had a problem set forth in the following. From the standpoint of further improving the efficiency of bit rate reduction coding, it is preferable to divide the entire screen into as many regions as possible to increase the number of small blocks included in each large block (fixed length block) to suppress deviation in the information amount of each fixed length block data. However, from the standpoint of ensuring good visual quality of the reproduced picture at the time of high speed reproduction, it is preferable to reduce the number of small blocks included in each large block (fixed length block) to ensure continuity of data on a screen for preventing mosaic deformation on the screen.
As shown in FIG. 2, for example, small blocks are derived one by one from the respective six regions A-F to form one large block (M1=A1, B1, C1, D1, El, F1), and code amount control is carried out for making the code amount for each large block constant to form a fixed length block. Then, recording onto a tape is carried out on the basis of this fixed length block (M1, M2, M3, M4, . . . ). If it is adapted that adjacent small blocks on a screen (for example, A1 and A2) are the two small blocks corresponding to each other in the two adjacent fixed length blocks (for example M1 and M2) on the tape, only 1/6 (for example, 5 small blocks) of the small blocks (for example, 30 small blocks) that can be reproduced continuously from a tape at the time of high speed reproduction is successive in each of the regions A-F. As described in the foregoing, the mosaic deformation is usually caused by generation of a large number of boundaries among blocks belonging to different fields. In order to present such mosaic deformation, therefore, it is required that small blocks belonging to the same field is reproduced on the screen as many as possible. If only 5 small blocks, corresponding to 1/6 of 30 small blocks which can be continuously reproduced on the screen, can be continuously reproduced in each of the regions A-F as described above, however, a large number of boundaries among blocks belonging to different fields are generated on the screen and mosaic deformation at the time of high speed reproduction cannot be prevented sufficiently.
Conversely, if the screen is divided into a smaller number, for example divided into two, 1/2 of the small blocks that can be reproduced continuously from a tape at the time of high speed reproduction can be successive on each of the regions, whereby visual quality of the picture at the time of high speed reproduction can be improved to some degree. However, in this case, there is a possibility of deviation in the information amount of each fixed length block to degrade the efficiency of bit rate reduction coding.
Therefore, the design parameter of a digital VTR, particularly the number of small blocks included in each fixed length block can be determined only by a compromise between the efficiency of bit rate reduction coding and the visual quality of a high speed reproduced picture, resulting in a problem that the design of a digital VTR significantly lacks degree of flexibility.